1. Field of the Invention
This invention generally relates to a method and apparatus for combatting ingress and multipath in a digital CATV return channel, and more particularly, to the use of a precoder in the transmitter of user equipment and an ingress rejection filter in the receiver of headend equipment.
2. Discussion of the Related Art
The Cable Television (CATV) return channel (alternatively referred to as the upstream cable channel) has become the focus of significant attention recently as is evidenced by IEEE 802.14 standardization activity. Typically, CATV was designed to provide a broadband access from a headend to a consumer in a tree network, i.e., a group of homes would be serviced by one node, which in turn gets serviced along with another set of such nodes, and so on, until the terminal node is the headend. A provision for a "return" transmission from the consumer to the headend was also provided. However, a lack of services on the CATV return channel has resulted in the design of a CATV network such that the CATV return channel is not as "clean" as the CATV forward channel. At this time, digital data promises to provide two-way services, for example, cable telephony, internet access, video conferencing, etc.. As a result of existing CATV networks and the corresponding CATV return channels, two-way digital data services are undesirably degraded and less than optimal.
The CATV return channel is characterized by the frequency band between 5-50 MHz. The CATV return channel is plagued by ingress noise resulting due to a poor shielding of cable used at different cable locations. In addition to ingress noise, multipath caused by microreflections can also deteriorate the quality of the channel for digital data transmission. As a result, the reliability of digital data transmitted over the CATV return channel is compromised and not ensured.
Currently, physical layer proposals submitted to the IEEE 802.14 deal with the problem of ingress noise by avoidance in several different ways. In a Frequency Agile Multi-Modem (FAMM) proposal, the method involves finding regions in the spectrum which do not have appreciable ingress noise and transmitting in those regions only. In multicarrier proposals Discrete Wavelet Multi-tone (DWMT) and Discrete Multi-tone (DMT), the method does not use carriers that are corrupted by ingress noise for data transmission. In a Code Division Multiple Access (CDMA) proposal, the method uses as spreading vectors only those eigenvectors of the ingress noise that have low variance. In each of the mentioned proposals, the methods disadvantageously sacrifice bandwidth in order to avoid ingress noise. In other words, the presence of ingress noise limits an available bandwidth for transmission since the proposed methods seek to avoid regions in the spectrum where the ingress noise is dominant.
Typical scenarios of digital data transmission over the CATV return channel may involve a combination of Frequency Division Multiple Access (FDMA) and Time-Division-Multiple Access (TDMA) schemes, as was observed in most of the Media Access (MAC) layer protocols proposed in the November 1995 meeting of IEEE 802.14, held in Montreal, Canada. FDMA is just a fancy name for subdividing the entire frequency band between 5-50 MHz into smaller equal-sized bands, typically of 6 MHz bandwidth. Within each 6 MHz band, TDMA is used, i.e., users that need to transmit data are assigned time slots within a TDMA frame. These time slots can be fixed for every user, which can be quite wasteful depending on the traffic model assumed for each user. Another strategy that can also be used is a reservation based policy in which the time slots are dynamically assigned to each user. Depending on which strategy is chosen, it can be assumed in general that transmission of data from slot to slot may or may not be from the same user (i.e., from the same TDMA transmitter). Thus, it can be seen that for the CATV return channel, a TDMA transmitter will transmit only in short bursts of time.
In order to assure reliability and maintain high integrity, digital TDMA transmission must be robust in the presence of ingress noise and multipath. This situation of digital TDMA transmission over a CATV return channel is complicated by the fact that each return path will have a different multipath characteristic. See for instance FIG. 1, in which a CATV system 10 having a return channel for multiple system users 12.sub.(1-N) is shown. System users 12.sub.(1-N) are each connected to headend 14 via a corresponding return channel, wherein each return channel has a different corresponding multipath characteristic (1-N). Fortunately, ingress noise can be assumed generally to have the same frequency domain characteristics during the period of one frame of transmission, since the headend receiver observes the sum of all ingress noise from all TDMA transmitters at any instant. It is, however, possible that the ingress will have somewhat different characteristics when different transmitters are active. This latter situation can occur since ingress is a result of the amount of atmospheric signals absorbed by an unshielded cable, which in turn depends on the voltage characteristics of the signal travelling through the cable.